Tuesday was the Rangers’ first practice in preparation for their opening-round playoff series against the Flyers, starting Thursday at the Garden, and a crucial cog in their forward corps, Derick Brassard, left the ice holding his lower back. If it only turns out to be a small tweak — which was the story coach Alain Vigneault was sticking to — then what was made most clear is that the Rangers can’t afford an injury like that.

Because replacing Brassard for the remainder of the hour-long practice was J.T. Miller, the 21-year-old second-year pro called up from AHL Hartford on Monday, after being demoted two weeks earlier with Vigneault’s harsh criticism chasing him all the way.

Miller settled between Mats Zuccarello and Benoit Pouliot in an attempt to fill out what inarguably has been the Rangers’ most consistently effective offensive line over the final couple months of the season, and also alongside those two on the second power-play unit.

Vigneault said he had no update on Brassard, who in the past has liked taking part in optional practices for the sake of staying fresh and had been off-ice since Saturday’s regular-season finale in Montreal. Vigneault was somewhat more declarative when asked if he thinks Brassard will be ready for Game 1.

“I would say yes,” Vigneault said.

Interpret as need be — and recognize Vigneault has a tendency to use the conditional “would say” quite often for no reason at all — but realize the point is the Rangers can ill afford to be shorthanded, especially at forward. They already are without top-six winger Chris Kreider, sidelined “indefinitely” following surgery on his left hand on March 28. Kreider skated by himself again on Tuesday with a soft cast on the hand. When Vigneault was asked about the possibility of Kreider being ready if this series goes to a Game 6 or 7, he punted.

“At this time, like I’ve said all along, it’s indefinite,” Vigneault said. “Until it changes, that’s what it is for now.”

Already leaning on 22-year-old Swede and first-year pro Jesper Fast to play on a line with Brad Richards and Carl Hagelin, the pressure would immediately be heaped on Miller, whose actions got Vigneault to say on April 3 that Miller could wind up being “a good minor league player” if he didn’t figure it out. The coach tried to clarify that pointed statement a bit on Tuesday.

“I would say those comments might have been interpreted by [the media] as pointed,” he said. “I would say to you that 95 percent of young players have to figure it out. … They’re either going to figure it out and become good NHL players, or they’re not, and they’ll be good minor league players. That comment I made of him could have been made of any young player.”

Miller, the No. 15-overall pick in 2011, has produced at the AHL level and failed to do so consistently in 56 career NHL games — 26 last season and 30 this one, with career totals of five goals and 10 points.

“Every time I get an opportunity I feel like I have to prove myself,” Miller said. “At the end of the day, I just have to play — try to be good at both ends of the ice and try to stay mistake-free. It’s hard, but at this time of year, you have to be.”

One of the Rangers’ keys to success during the final part of the regular season, and one of the main reasons they made the playoffs — next to the reincarnation of Henrik Lundqvist — was their ability to roll out three offensive lines, and back it up with a solid checking line as the fourth.

“That’s one of the reasons we called up J.T.,” Vigneault said, “because we feel if one part of any line goes down that he has the potential and the skill-set to come in, and, for an amount of time, be able to take that spot.”